1. Field of the Invention
This invention relates to the field of foldable or elastometric implantable intraocular lenses composed primarily of silicone rubber and, more particularly, pertains to a method of extending the spectral range through which these lenses block ultraviolet radiation.
2. Description of the Prior Art
Plastic materials have been rendered resistant to passage of ultraviolet radiation by the addition of certain chemical compounds. Typical compounds used to impart ultraviolet screening properties fall primarily into one of two classes, hydroxybenzophenones or hydroxybenzotrizoles. These materials are effective in plastics in blocking transmission of 90% of the ultraviolet radiation below a wavelength of 400 nm when used at concentrations below 1% in a plastic sheet 1 mm thick.
Many of the same ultraviolet screening agents may also be mixed into silicone rubber used to produce foldable intraocular lenses. However, if these agents are mixed into the silicone rubber before it is cured by means of heat and pressure, they may cause undesirably weak and soft lenses. Additionally, when silicone lenses are treated to prevent fogging when immersed in water, some of the screening agents may be lost by leaching from the silicone matrix. Lastly, the commonly used screening agents do not absorb light at 400 nm when used at practical levels in silicone rubber.
The shortcomings of excessive yellowing and reduced ultraviolet absorption at 400 nm are minimized in the lenses of the present invention which are made using diffusion to incorporate into the lens a combination of an ultraviolet screening agent and an optical brightener after it has been shaped and cured.